1. Technical Field
This invention relates generally to optical imaging systems for measuring part coordinates and more particularly to such systems having a controller for automatically presetting the level of one or more illumination sources to enhance the accuracy of the measurement.
2. Description of the Related Art
Optical imaging systems for measuring part coordinates have the advantage of allowing measurements to be made without contacting the part being measured. Optical imaging systems have the complication, however, of being sensitive to proper lighting of the part being measured. If light levels are too low, the signal created by the imaging system may be too low compared with noise levels to allow accurate measurement. If light levels are too high, saturation of the image will cause measurement errors because the image blooms and edges in the image move from their true locations.
Modern optical measurement systems often have multiple illumination sources, e.g. co-axial lights, backlights, and oblique lights. Each of the illumination sources must be set at an appropriate illumination level to produce accurate measurements.
In addition to the increasing number of illumination sources that may be used and adjusted simultaneously to make accurate measurements, modern measurement systems often include zoom lenses. When the magnification of a zoom lens is changed, the light collection efficiency of the lens usually changes. This causes a change in the portion of the light from the illumination source(s) that contributes to the image. Consequently the levels of the illumination sources usually need to be adjusted whenever the lens magnification is changed to achieve optimum measurement accuracy.
From measurement system to measurement system the sensitivity of the camera and the gain of the signal processor may differ. Even for a given machine, sensitivity of the camera and the signal processing gain may be changed from time to time to optimize certain optical measurements. These differences in detection sensitivity also usually result in a different illumination light source intensity being required for optimum measurement accuracy.
Traditionally, automatic part measurement programs for particular parts have been developed in a learning mode. The operator puts an optical measurement system into a manual mode, selects the sequence of measurement steps he wants for that part, including positioning the part and selecting the appropriate zoom magnification for each measurement, and manually adjusts the light levels of the illumination sources until he gets good measurement results. The operator then saves the parameters of each measurement step along with the associated light levels, most often in a computer that will control subsequent automatic measurements. This manual programming method requires a highly skilled operator.
U.S. Pat. No. 6,627,863 to Wasserman describes a method of semi-automatically determining the appropriate levels of the lights in a particular measurement system. This method compares measurements of an actual object made on the machine being programmed and simulated images to set the appropriate illumination light source levels. While this method may reduce the required skill level of the operator and perhaps the programming time, it still ties up a valuable measurement system during programming. The system cannot be used for measuring parts while it is being used for developing part programs.
It is desirable to develop measurement programs in an offline mode, i.e., with the measurement software running on a computer that is not attached to an actual measurement machine. U.S. Pat. No. 7,092,860, also to Wasserman, mentions an automated, offline system. It further mentions a lighting model used to automatically set the parameters for the various lights in the system, but gives no indication as to how such a model could be developed. Developing such a model incorporating multiple light sources and zoom lenses is difficult. The '860 patent does not describe any method for developing the model. It does not describe the steps required. It does not even give an example of the development of such a model.
U.S. Pat. No. 5,389,774 to Gelman and Davis describes an apparatus for calibrating a zoom lens and a calibration method that generates data used for setting a zoom lens to a predetermined magnification setting. The present inventors have discovered that that same data can be used when combined with other data to determine the appropriate compensation of illumination sources in accordance with an aspect of this disclosure.
The '774 patent describes a method of calibrating the magnification of a zoom lens. The calibration is carried out on each combination of a measurement machine, zoom lens, and camera. Referring to that patent's FIG. 2 the zoom lens magnification is changed by driving a servo motor 23 to turn a spur gear 26 attached to the motor shaft. The spur gear 26 engages a ring gear 24. The ring gear 24 surrounds and is attached to the adjustable lens barrel 25. When the lens barrel 25 turns, the magnification of the zoom lens is changed. To calibrate the zoom lens for enabling an operator to return the lens to a predetermined desired zoom magnification, a reticle image is projected at the selected magnification and via the zoom lens and a video camera to a microprocessor, which electronically stores that particular image of the reticle for subsequent use when it is desired to have the now-calibrated magnification reestablished. To reestablish the calibrated magnification at a later time, a new image of the reticle is projected to the CPU and via the CPU to a video screen. The previously recorded image of the reticle is also projected onto the video screen by the CPU and is mathematically compared with the new reticle image. The zoom lens is adjusted until the two images are coincident at which time the previously selected magnification will have been reestablished. As part of the calibration process the '774 patent records the reticle light intensity for each magnification at which the zoom lens is calibrated and the x,y,z coordinates. The reticle light intensity is adjusted during calibration to a satisfactory level to produce a visible image. To return the zoom lens to a pre-calibrated position the reticle intensity is reset to the level saved during the calibration process.
The technique described in the '774 patent is widely used and the parameters stored during calibration of a zoom lens on a particular optical inspection system are commonly available to the operator of the system. The inventors have discovered a way to use this data to simplify the programming of optical imaging systems.